docs: prevent smartphones from being too smart (docbook)

Add HTML <meta> tag to not auto-create telephone number links on mobile browsers also in docbook processed documents.

Reference: https://github.com/networkupstools/nut/issues/78

XSL files source:
- https://github.com/asciidoc/asciidoc/blob/master/docbook-xsl/common.xsl
- https://github.com/asciidoc/asciidoc/blob/master/docbook-xsl/xhtml.xsl
- https://github.com/asciidoc/asciidoc/blob/master/docbook-xsl/chunked.xsl

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For the docbook part I had to add the xsl files: while this adds some weight to the repo, we could do nice things in future with them to personalize even further the output.